Shallow hydrocarbons in Lower Austria: A drilling hazard and a valuable exploration tool
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© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Austrian Journal of Earth Sciences Volume 106/1 Vienna 2013
Shallow hydrocarbons in Lower Austria: A drilling
hazard and a valuable exploration tool_______________
*)
Jan MAYER & Reinhard F. SACHSENHOFER
KEYWORDS
Petroleum System
Molasse Basin
Vienna Basin
Department Applied Geosciences and Geophysics, Montanuniversität, Peter-Tunner-Str. 5, A-8700 Leoben, Austria; Gas Show
Oil Show
*)
Corresponding author, reinhard.sachsenhofer@unileoben.ac.at Seepage
Abstract
The presence of shallow gas constitutes a drilling hazard (several gas blow outs occurred recently in shallow wells in Lower Austria),
but provides also a valuable tool for hydrocarbon exploration. 65 gas shows, 12 oil and gas shows, and 18 oil shows are known in
Lower Austria at the surface or at shallow depth (< 300 m below surface; b.s.). Most gas shows are located in the Vienna Basin
(39) followed by the Molasse Basin (28) and the Flysch Zone (13). Most oil (and gas) shows occurred in the Vienna Basin (11), the
Northern Calcareous Alps (8) and the Flysch Zone (7). In addition five gas fields are present in Lower Austria, where at least one
reservoir horizon is at a depth of less than 300 m b.s.______________________________________________________________
Shallow hydrocarbon shows in the Vienna Basin are connected to the foot- and hangingwall of the Steinberg, Pirawarth and Leo-
poldsdorf faults and hydrocarbon fields in the central part (Matzen, Fischamend) of the basin. There are only a few oil and gas
shows without obvious connection to known deposits. Some hydrocarbon shows contain hydrogen sulfide, which is probably linked
to the presence of Permo-Triassic gypsum/anhydrite in underlying units. The gas shows in the Molasse Basin east of the Bohemian
Massif coincide with a hydrocarbon field and coal-bearing Permo-Carboniferous graben sediments. Numerous oil and gas shows
are located along a north-south trend in the southern Waschberg Zone and the northeasternmost Alps (Flysch Zone, Northern Cal-
careous Alps). The regional distribution suggests an origin from Gresten Formation of Doggerian age. A hydrogen-rich gas show
near the southern margin of the Northern Calcareous Alps is linked to evaporitic Permo-Triassic rocks near the base of the Nor-
thern Calcareous Alps. Most of the gas shows in the Molasse Basin west of the Bohemian spur are located in an area close to the
river Enns. With the current data it is impossible to decide whether the gas is of biogenic origin or formed by degradation of pre-
existing oil generated from Oligocene source rocks (Schöneck Formation). Whereas oil (and gas) shows occur in the Klippen Belt
and the Inneralpine Molasse, pure gas shows, partly with high pressure, have been recorded in the Flysch Zone. Oil shows in the
Gresten Klippen Belt are probably related to coals of the Jurassic Gresten Formation. An oil seep initiated drilling of the well Ur-
mannsau 1 in the Northern Calcareous Alps, which encountered non-commercial oil and gas. Biomarker data suggests a source
rock with relatively low maturity deposited in an anoxic environment and a significant contribution of terrestrial plants.___________
The knowledge of the areal distribution of shallow gas accumulations is essential to define zones with a very high risk to encounter
gas at shallow depths.
Das Vorkommen von oberflächennahem Gas stellt ein Bohrrisiko dar (mehrere Gasausbrüche traten in den letzten Jahren in Seicht-
bohrungen in Niederösterreich auf). Gleichzeitig sind seichte Kohlenwasserstoffe aber auch ein wertvolles Hilfsmittel für die Explo-
ration. 65 Gas-, 12 Öl- und Gas- und 18 Ölanzeichen sind in Niederösterreich an der Oberfläche und bis in eine Tiefe von 300 m
unter Geländeoberkannte (u. GOK) bekannt. Die meisten Gasanzeichen traten im Wiener Becken (39), gefolgt vom Molasse Becken
(28) und der Flysch Zone (13) auf. Die meisten Ölanzeichen (plus Öl- und Gasanzeichen) befinden sich im Wiener Becken (11), in
den Nördlichen Kalkalpen (8) und der Flysch Zone (7). Zusätzlich treten 5 Gaslagerstätten auf, bei denen zumindest ein Gashori-
zont in einer Tiefe von weniger als 300 m u. GOK liegt._____________________________________________________________
Seichte Kohlenwasserstoffanzeichen im Wiener Becken stehen im Zusammenhang mit dem Steinberg Bruch, der Pirawarther Bruch-
zone und dem Leopoldsdorfer Bruch. Sie treten sowohl in der Liegend- als auch in der Hangendscholle auf. Weitere seichte Kohlen-
wasserstoffanzeichen befinden sich im Bereich von Lagerstätten im Beckenzentrum (Matzen, Fischamend). Es ist nur bei einer ge-
ringen Anzahl von Öl- und Gasanzeichen keine Verbindung zu bekannten Lagerstätten erkennbar. Einige Kohlenwasserstoffanzei-
chen beinhalten Schwefelwasserstoff, der vermutlich in Verbindung mit dem Auftreten von permo-triassischem Gips/Anhydrit im Un-
tergrund steht. Die Gasanzeichen im Molasse Becken östlich des Sporns der Böhmischen Masse stehen in Verbindung mit einer
Kohlenwasserstofflagerstätte und kohleführenden permo-karbonen Grabensedimenten. Zahlreiche Öl- und Gasanzeichen befinden
sich entlang eines Nord-Süd orientierten Bereichs in der südlichen Waschberg Zone und den nordöstlichsten Alpen (Flysch Zone,
Nördliche Kalkalpen). Die regionale Verbreitung deutet eine Herkunft aus der Gresten-Formation des Doggers an. Ein wasserstoff-
reiches Gas in der Nähe des Südrandes der Nördlichen Kalkalpen steht in Verbindung mit evaporitischer Permo-Trias im Bereich
der Basis der Nördlichen Kalkalpen. Die meisten Gasanzeichen im Molasse Becken westlich des Sporns der Böhmischen Masse
befinden sich nahe der Enns. Mangels Analysedaten ist es unmöglich zu entscheiden, ob das Gas biogener Herkunft ist, oder durch© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Jan MAYER & Reinhard F. SACHSENHOFER
Biodegradation eines ursprünglichen Öls entstanden ist, das in oligozänen Muttergesteinen gebildet wurden. Öl- und Gasanzeichen
treten in der Klippenzone und in der Inneralpinen Molasse auf. Reine Gasanzeichen, teilweise mit hohen Drücken, wurden in der
Flyschzone bekannt. Ölanzeichen in der Grestener Klippenzone stehen wahrscheinlich im Zusammenhang mit der jurassischen Gres-
tener Kohle. Ein Ölaustritt in den Nördlichen Kalkalpen gab Anlass zur Bohrung Urmannsau 1, welche nicht-ökonomische Kohlen-
wasserstoffe antraf. Biomarker-Daten belegen ein gering reifes Muttergestein, welches in anoxischem Milieu mit signifikantem Ein-
trag von terrestrischen Pflanzen abgelagert wurde._________________________________________________________________
Die Kenntnis der Verteilung der seichten Kohlenwasserstoffanzeichen ermöglicht eine Abgrenzung von Zonen mit sehr hohem
Risiko für das Antreffen von seichten Gasakkumulationen.___________________________________________________________
1. Introduction
On the 25th of July 2011 a bore-
hole, designed as a downhole heat
exchanger (DHE), erupted due to a
gas blow out in a depth of 98.5 m
below surface (b.s.) in St. Pantaleon-
Erla (N St. Valentin; Fig. 1, 5). The
blow out was only brought under
control by OMV on the next day
(http://noev1.orf.at/stories/529020).
A similar gas-blow out occurred on
the 29th of July 2005 in Herzograd/
St.Valentin. During this blow out two
drillers were seriously injured be-
cause of the self-ignition of the gas.
Figure 1: Geological map of Austria and area of interest.________________________________
To assess the danger of blow outs
in shallow wells (© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Shallow hydrocarbons in Lower Austria: A drilling hazard and a valuable exploration tool
mation (Autochthonous Mesozoic; Ladwein, 1988) and migra- ted in the present paper. The literature survey also conside-
ted along faults through the Alpine thrust complex. Gas fields red natural gas fields, where at least one reservoir horizon is
in the central basin SW of Orth contain biogenic gas (Schoell, at a depth of less than 300 m b.s. (Brix and Schultz, 1993),
1984; Ladwein, 1988). The Zwerndorf Field contains thermo- and blow outs of hydrocarbon exploration wells (Spörker and
genic gas with an admixture of biogenic gas. Gas/condensate Logigan, 1988), because they might have caused gas migra-
and oil produced from autochthonous Mesozoic sediments tion into shallow horizons._____________________________
beneath the Flysch Zone (Höflein Field) and the Waschberg In addition, the reports of 2595 counterflush (CF) boreholes,
Zone may have a contribution of hydrocarbons generated in drilled during the 1930ies and 40ies for structural investiga-
the coal-bearing Gresten Formation of Doggerian age (Krato- tions, were inspected for hydrocarbon shows._____________
chvil and Ladwein, 1984; Ladwein, 1988; Sachsenhofer et al., Finally, a total of 164 drilling companies, geological enginee-
2006). Gas of mixed biogenic / thermogenic origin has been ring bureaus, experts, and district commissions (Bezirkshaupt-
found in shallow Miocene horizons in the Roseldorf Field mannschaften) were asked to provide data on gas and oil shows.
(Waschberg Zone). Gas in the Molasse Basin is of biogenic Fearing negative publicity, some companies obviously withheld
origin (Schoell, 1984).________________________________ the needed information, or provided it only after signing confi-
dentiality agreements.________________________________
3. Data acquisition Because of the confidentiality issues, only published infor-
The study is based on an extensive literature survey. The mation is added in the reference list. The focus of the study
most important reference was Brix (1993), who listed 40 hy- is Lower Austria. Hydrocarbon shows beyond its border have
drocarbon shows in Lower Austria (incl. Vienna). Brix (1993) also been recorded. As the focus was on Lower Austria the
includes numerous unpublished references, which are not lis- data acquisition in those additional areas is not complete.___
Figure 2: Geological map of Lower Austria with position of gas, oil and gas, and oil shows. Rectangles mark positions of Figs. 3 and 5. The posi-
tion of the Steinberg and Leopolsdorf fault systems are shown after Wessely (2006)._____________________________________________________© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Jan MAYER & Reinhard F. SACHSENHOFER
4. Results
4.1 Hydrocarbon shows
65 gas shows, 12 oil and gas
shows, and 18 oil shows are known
to occur in Lower Austria at the sur-
face or at a depth of less than 300 Table 1: Numbers of hydrocarbon shows in different geological units of Lower Austria. Numbers
m b.s. (Table 1, Fig. 2). Another 15 of shows outside of Lower Austria are shown in brackets.___________________________________
gas shows (10 in Vienna, 5 in Up-
per Austria, 1 in Styria), a single oil
show (asphalt) in Upper Austria and one oil show in Vienna Most hydrocarbon shows are aligned along the fault systems
occurred close to the border of Lower Austria._____________ (Steinberg, Pirawarth, Leopoldsdorf faults) bounding the de-
Most gas shows are located in the Vienna Basin (39) follo- pocenters of the Vienna Basin towards the west.__________
wed by the Molasse Basin (28) and the Flysch Zone (13). Most A high number of oil and gas shows and also gas fields
oil (plus oil and gas) shows occurred in the Vienna Basin (11), with shallow reservoir horizons (Althöflein: Badenian; 225-
the Northern Calcareous Alps (8)
and the Flysch Zone (7)._________
4.2 Shallow gas fields
Shallow (< 300 m b.s.) gas fields
have been produced in the Vienna
Basin (Althöflein, Gösting, Oberlaa)
and the Waschberg Zone (Merkers-
dorf, Roseldorf) (Fig. 3)._________
4.3 Blow outs at hydro-
carbon exploration wells
Thirteen blow outs occurred be-
tween 1935 and 1952 at hydrocar-
bon exploration wells (Spörker and
Logigan, 1988) in Lower Austria
(Fig. 3). Gas migration into shallow
horizons caused by a blow out is
proven for the Mühlberg 1 well,
where a major blow out in the year
1941 caused a successor blow out
in an abandoned shallow CF well
located about 250 m east of Mühl-
berg 1.
5. Discussion
5.1 Geological setting
of hydrocarbon shows
5.1.1 Gas and oil shows
in the Vienna Basin______
The discovery of the gas seepage
in Gbely (Egbell) in Slovakia, detec-
ted in 1912 by Jan Medlen, led to
the detection of the first oil reservoir
in the Vienna Basin (Wessely, 2006).
There is an obvious connection be-
Figure 3: Position of hydrocarbon fields (Arzmüller et al., 2004), hydrocarbon shows and blow
tween hydrocarbon shows, gas and outs in the eastern part of Lower Austria. The distribution of potential source rocks (Permo-Carbonife-
oil fields and major fault zones (Fig. 3). rous; Gresten Fm., Mikulov Fm.) is shown (Kröll and Wessely, 2001).__________________________© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Shallow hydrocarbons in Lower Austria: A drilling hazard and a valuable exploration tool
(Sarmatian; 714-730 m b.s.) pro-
duced gas from the Leopoldsdorf
Fault System, suggesting that
the Oberlaa gas migrated along
this major fault system, which is
currently under extension and a
potential migration pathway. Oil
shows in the St. Marx 1 well sug-
gest that the gas is thermogenic
in origin.
Apart from the basin margin faults,
gas shows with a strong smell of hy-
drogen sulfide have been recorded
from the central high of the Vienna
Basin above the giant Matzen Field.
Notably, no hydrocarbon shows have
been recorded so far above the Ader-
klaa and Zwerndorf fields.________
In contrast, gas (with H2S) was de-
Figure 4: N-S trending 150 m long seismic section (time domain in ms) along a 35-m-deep and tected above fields with biogenic
20-cm-thick gas bearing horizon in the Oberlaa High. Despite its low thickness, it causes an amplitude
gas in the central zone of the Vien-
anomaly (bright spot). This shows that the extension of shallow gas can be detected using high reso-
lution reflection seismic even in urban areas (Schreilechner and Eichkitz, 2013)._________________ na Basin south of river Danube (e.g.
Enzersdorf-Fischamend, S of Orth).
Here, active strike slip faulting along
232 m b.s.; Gösting: Upper Pannonian) occur in the foot- the basin center fault caused an updoming of Sarmatian sedi-
wall and in the hangingwall of the Steinberg Fault (Friedl, ments and might be responsible for hydrocarbon migration
1937; Streintz, 1933; Brix, 1993).____________________ into shallow horizons.________________________________
Hydrocarbon shows occur in the footwall (Kronberg – Pira- Apart from the hydrocarbon shows mentioned above, which
warth High) and in the hangingwall (above Pirawarth Field) can be easily linked to underlying hydrocarbon fields, there
of the Pirawarth Fault._____________________________ are only a few oil and gas shows without obvious connection
Several gas shows, including a blow out in a shallow explo- to known deposits.__________________________________
ratory well for the subway in Vienna in 2009 (Schreilechner Considering the origin of the hydrocarbon deposits (Ladwein,
and Eichkitz, 2013; Fig. 4), and the small Oberlaa Field (Ba- 1988), it is likely that most of the oil and gas shows represent
denian, 262-266 m b.s.) have been recorded in the footwall hydrocarbons generated from the Malmian Mikulov Formation
of the Leopoldsdorf Fault System (Oberlaa High). The gas (Fig. 7). However, considering the biogenic origin of gas south-
fields St. Marx (Badenian; 410-427 m b.s.) and Favoriten west of Orth, a similar origin is assumed for gas shows in this
area (e.g. Fischamend). The vertical
arrangement of a mature source rock
(Mikulov Fm.), a high number of hy-
drocarbon reservoirs in different stra-
tigraphic positions above the hydro-
carbon kitchen, and hydrocarbon
seeps are typical for overcharged,
vertically drained petroleum systems
(Demaison and Huizinga, 1994).___
Many gas shows in areas under-
lain by the Northern Calcareous Alps
(Matzen, Oberlaa) and the Central
Alps (Fischamend) contain hydrogen
sulfide. In contrast, hydrogen sulfi-
de has not been recorded from gas
shows in the northern part of the Vi-
enna Basin underlain by the Flysch
Figure 5: Geological map of the western part of Lower Austria together with the positions of gas,
oil and gas, and oil shows. The position of some gas fields in Upper Austria and the eastern boundary Zone. This observation suggests a
of Oligocene source rocks (Sachsenhofer and Schulz, 2006) are also shown.____________________ genetic link to the presence of Per-© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Jan MAYER & Reinhard F. SACHSENHOFER
mo-Triassic gypsum/anhydrite and sour gas in basement units. though an origin from the Werfen Formation cannot be exclu-
ded, the gas accumulation was probably charged by evapori-
5.1.2 Gas and oil shows in the eastern tic Permo-Triassic rocks near the base of the Northern Calca-
part of the Molasse Basin, the Wasch- reous Alps. Similar gas exhalations are frequently encounte-
berg Zone, and the eastern sector of red in Alpine salt deposits (Schauberger, 1960).___________
the Alps
Only two gas shows are known from the Molasse Basin east 5.1.3 Gas and oil shows in the Molasse
of the Bohemian Massif. One might be related to the small Basin west of the Bohemian Spur________
Laa Field, the other one (near Mailberg) coincides with Permo- A high concentration of gas shows, partly with relatively high
Carboniferous graben sediments beneath the Molasse Zone. pressure (> 5 bar), is found in the Molasse Basin close to the
Although analytical data are not available, we speculate that river Enns and east of St. Valentin (Holzleitner, 1926; Fig. 5).
the latter show might be related to coal-bearing Carboniferous For the discussion of their origin, the following observations
rocks (Fig. 3). This would resemble the situation in Switzer- made close to the border in Upper Austria are important:____
land, where the country´s only gas field (Entlebuch) is char- The boreholes Steyr 1 (245-247 m b.s.) and Harmannsdorf
ged by Permo-Carboniferous coal measures (Veron, 2005).__ 2 (145 m b.s.) drilled subeconomic shallow dry (biogenic?)
Two oil shows and one gas show occur in the southern part gas (C1/[C2+C3] = 425-575) in Lower Miocene horizons.___
of the Waschberg Zone (Vetters, 1921). Hydrocarbon deposits Slightly deeper gas deposits (550-730 m) in Eocene and
in this area occur at depths of up to 4 km in the autochthonous Upper Cretaceous horizons (e.g. Thann-Teufelsgraben,
Mesozoic and in shallow, deformed Cenozoic (and Cretace- Harmannsdorf) are underlain by a heavy, biodegraded oil
ous) sediments. The latter include gas accumulations at Mer- (Gratzer et al., 2011). Based on (rare) isotope and compo-
kersdorf (Ottnangian; 200-255m b.s.) and Roseldorf (250 to sitional data, Reischenbacher and Sachsenhofer (2011)
>1550 m b.s.; Brix, 1993)._____________________________ argue that these gas deposits formed by biodegradation of
Further to the south, gas and oil shows have been detected pre-existing oil deposits.___________________________
in the eastern part of the Flysch Zone (e,g. hydroelectric power Uplift of the easternmost part of the Upper Austrian Molas-
station Greifenstein; Vetters 1921; Waagen, 1931; Streintz, se Basin resulted in westward tilting of Molasse sediments
1933; Schwingenschlögl, 1984) and are often related to faults. and controlled migration paths (Gusterhuber et al., 2012)._
The gas dryness (C1/[C2+C3] = 44-
49) suggests a thermal origin (un-
published data from 2008). Oil shows
occur in the easternmost sector of
the Northern Calcareous Alps (Brix,
1993).
The regional distribution of shallow
gas and oil shows in the Waschberg
Zone and near the eastern margin
of the Flysch Zone / Northern Calca-
reous Alps suggests an origin from
the Doggerian Gresten Formation
(Fig. 7). The hypothesis is further
supported by petroleum deposits in
the Gresten Formation itself and in
the Autochthonous Mesozoic sedi-
ments overlying it. The origin of gas
and oil shows in the Flysch Zone
east of the areal extent of the Gres-
ten Formation (Fig. 7) is less obvi-
ous.
A gas show was detected in 1968
in in the Schneealpen tunnel near
the southern margin of the Northern
Calcareous Alps (Fig. 2; Gattinger,
1973). The hydrogen-rich gas esca-
ped from anhydrite within the Lower
Figure 6: Chromatogram (Total Ion Current) of the saturated hydrocarbon fractions and mass
chromatogram (m/z 191) for triterpanes of the Urmannsau rock extract. n-Alkanes are labeled accor-
Triassic Werfen Formation and was ding to their carbon number. Pr. = pristane, Ph = phytane, Std. = deuterated tetracosane. Tm = 17a(H)
relatively dry (C1/[C2+C3] = 118). Al- trisnorhopane.© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Shallow hydrocarbons in Lower Austria: A drilling hazard and a valuable exploration tool
Without molecular and isotopic data of the gas shows, it is shows in the Inneralpine Molasse, the Klippen Belt and the
impossible to decide whether the gas detected in Lower Aus- Flysch Zone. In a similar tectonic situation a gas explosion oc-
tria is of biogenic origin or formed by degradation of a pre-exis- curred 18 km east of Scheibbs in Glosbach (Götzinger, 1931
ting oil deposit charged by Oligocene source rocks (Schöneck a,b). The explosion occurred within Flysch sediments, which
Formation). The eastern limit of this source rock is indicated are in close contact to Inneralpine Molasse sediments and
in Figure 5 and possible migration directions are shown in Fi- the Klippen Belt.____________________________________
gure 7. Thus, geochemical data is needed urgently to under- Whereas oil (and gas) shows occur in the Klippen Belt and
stand the petroleum potential of the Molasse Basin in Lower the Inneralpine Molasse, pure gas shows, partly with high pres-
Austria. sure, have been identified in the Flysch Zone. A gas accumu-
A gas show close to Amstetten was encountered while dril- lation encountered in a tunnel NE Scheibbs (Hochpyhra tun-
ling for coal in the middle of the 19th century (Vetters, 1921, nel) was burning for 14 days (Jenikowsky, 1934; Vetters, 1937,
1937). 1938; Brix and Schultz, 1993). The source rock of the gas re-
mains unknown.____________________________________
5.1.4 Gas and oil shows in the Inneral- Oil shows (including asphalt) occur in rocks of the Gresten
pine Molasse, the Klippen Belt and the Klippen Belt between Maria Neustift in the west and Gresten
Flysch Zone of western Lower Austria__ in the east (Fig. 5; Petrascheck, 1924; Sigmund, 1937; Vet-
Oil shows in the “Inneralpine Molasse” west of Scheibbs re- ters, 1938; Brix, 1993). Probably they are related to the Juras-
sulted in the drilling of the 461 m deep Rogatsboden 1(1a) ex- sic Gresten coal, which is often bituminous and reaches oil
ploration well (Prey, 1957). The well encountered gas and oil window maturity (Sachsenhofer, 1987).__________________
Figure 7: Geological map of Lower Austria together with the positions of gas, oil and gas, and oil shows. The supposed origin, migration pathways
and position of the shallow gas and oil shows are shown. The eastern boundary of Oligocene source rocks (Sachsenhofer and Schulz, 2006) and the
distribution of potential source rocks (Permo-Carboniferous; Gresten Fm., Mikulov Fm.; Kröll and Wessely, 2001) are also shown.________________© Österreichische
Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.biologiezentrum.at
Jan MAYER & Reinhard F. SACHSENHOFER
5.1.5 Gas and oil shows in the Northern The high density of gas and oil shows in its easternmost
Calcareous Alps of western Lower Aus- segment proves an especially high risk. Fault zones are
tria potentially dangerous._____________________________
There are several oil (and gas) shows in the Northern Cal- The Northern Calcareous Alps are an area with a low po-
careous Alps in the western part of Lower Austria (Brix, 1993; pulation density. It is possible that this results in an under-
Fig. 5). The oil from the most prominent one, near Urmanns- estimation of the gas hazard. In any case, the possibility
au, was used by monks from the nearby charterhouse Ga- of the appearance of shallow hydrocarbons always has to
ming as fuel for lamps and for pharmaceutical purposes (Rut- be taken into account. The risk is especially high in areas
tner, 1963; Wessely, 2006). The oil seep led to the drilling of with evaporitic Permo-Triassic successions.____________
the hydrocarbon exploration well Urmannsau 1 (Schaffer 1941; No shallow gas has been recorded so far from the Bohe-
Kröll and Wessely, 1967). Although the well encountered oil mian Massif, the Greywacke Zone and the Central Alps.__
and gas (e.g. at 274 m depth), no commercial hydrocarbons The above compilation shows that several zones with a very
were found. The oil showed a sulfur content of 0,69% (Suida high shallow gas hazard exist in Lower Austria. To further as-
and Pöll, 1933). The Urmannsau oil seep is no longer visible, sess the shallow drilling hazard, we suggest to test different
but Quaternary conglomerates with a strong petroleum smell techniques (e.g. surface geochemistry, high resolution seis-
occur near the river Erlauf. These gravels have been extracted mics) to delineate the extension of the shallow gas accumula-
and analyzed by R. Gratzer (Fig. 6). A significant odd-even tions in different geological settings (e.g. Oberlaa High, Mo-
predominance and a pristane/phytane ratio of 0.7 suggest a lasse Basin near St. Valentin)._________________________
source rocks with relatively low maturity deposited in an anoxic
environment and a significant contribution of terrestrial plants. 6. Conclusions
Notably the extract is not (heavily) biodegraded. Obviously Shallow (< 300 m b.s.) hydrocarbon shows are known in Lo-
additional investigations are needed to decide whether the wer Austria in the Molasse Basin, the Flysch Zone, the Wasch-
Urmannsau oil was generated from Oligocene Molasse sedi- berg Zone, the Vienna Basin and the Northern Calcareous Alps.
ments beneath the Alps, or from source rocks within the Nor- The position of the shallow oil and gas shows, their supposed
thern Calcareous Alps._______________________________ origin and migration pathways are shown in Figure 7._______
A shallow gas risk exists in all geological units with known
5.2 Areas with geological hazard of shallow hydrocarbon shows. The risk is especially high in the
hydrocarbons in shallow wells Vienna Basin above known hydrocarbon deposits and along
Although the information on shallow gas is certainly biased major faults, in the northeasternmost part of the Flysch Zone,
by the population density and the number of observation points and in the Molasse Basin and Flysch Zone in the western
(e.g. number of water wells and shallow boreholes), zones with part of Lower Austria. ________________________________
variably high risk for drilling shallow gas charged sediments
can be recognized. It will be the responsibility of the authorities Acknowledgments
to specify drilling techniques, safety equipment and special The authors thank OMV and the Federal State of Lower
training for drillers in areas with a high risk. Furthermore, it Austria for the permission to publish the study results. OMV
should be obligatory for drilling companies to communicate provided permission to screen CF boreholes. K. Grösel and
gas shows to the authorities.__________________________ G. Wessely significantly contributed to the present study with
Vienna Basin: There is a high risk to drill natural shallow their regional knowledge. R. Gratzer (Leoben) kindly provided
hydrocarbons above oil and gas fields. In addition, blow geochemical results from the Urmannsau oil show. _________
outs during the early days of hydrocarbon exploration (be-
fore 1955) may have caused gas migration into shallow
horizons (e.g. Mühlberg, Matzen). A high risk also exists in
the footwall of the major fault systems (Steinberg, Pirawarth- References
Hochleiten, Leopoldsdorf faults) bordering the depocenters Arzmüller, G., Buchta, S., Ralbovsky, E. and Wessely, G., 2004.
to the west (e.g. Kronberg – Pirawarth High SW Pirawarth, The Vienna Basin. In: Golonka, J. and Picha, F.J. (Eds.), The
Oberlaa High). Continuing extension in these areas facili- Carpathians and Their Foreland: Geology and Hydrocarbon
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Received: 12 December 2012
Accepted: 20 April 2013
Jan MAYER & Reinhard F. SACHSENHOFER*)
Department Applied Geosciences and Geophysics, Montanuniversität,
Peter-Tunner-Str. 5, A-8700 Leoben, Austria;_____________________
*)
Corresponding author, reinhard.sachsenhofer@unileoben.ac.at_____You can also read
